Can cable TV/Internet splitter affect signal "upstream"?

I hope this is the right forum category for this question:

I recently split a cable that had been coming to my modem. Now that cable goes to the modem and a dual TV tuner in a computer. But now
I feel that my picture on another computer with a dual TV tuner is degraded. But this comes off a different cable that is split near where the cable comes to the house from outside, so well before the new split.

Does that degrade signal on the other computer make sense. And, if so, would a bidirectional amplifier placed where the new splitter is inserted help upstream and downstream? Finally, is it better to put an amplifier prior to all splits where all cables come together? (I could use with some recos on amplifiers if that is the case> )

Thanks for any help!

Larry

Thanks. I'll start by calling them. Good advice.